Sit in a slot? That sounds awful close to an insult to me. But I'm gonna give you the benefit of the doubt.
Most OBC have inline fuses - have you checked them? Have you metered the batteries and charger? Meter the battery, record results, plug up charger, meter the leads connected to the battery and record the results.
When was the last time you checked the water in those batts? Never mind, I know the answer. Top them off with distilled water only, then recheck everything.
If you have to swap them out (what's the age by the way) ignore the stewpit "slots" - they're called battery trays ya idjit. If you have room for 2 group 31s - do it and change out the slots. Bwahahahahaha. So, measure to see if you can get them in there, if not, go to 29s, if not, take a hammer and beat enough fiberglass out of the way to squeeze the 29s in there. A pair of 27s just ain't gonna cut it for ya long term.
You need a minimum of a group 27 cranker and I'd rather see a 29 in there BTW.
Wannabe...
